The present invention relates to an optical system of the type used for inspection and observation of defects and foreign substance-induced defects in the surface of a pattern that is formed on a substrate through a thin film preparation process, such as a semiconductor production process and a flat-panel display manufacturing process. The invention also relates to a method of inspecting for defects using the optical system, an apparatus therefor, and a method for effectively using obtained inspection information.
Japanese Patent Laid-Open No. 9-218165 discloses an optical system for inspecting photomasks. This optical system operates as a Mach-Zehnder interferometer in which two sheared beams of light are applied to a mask surface. Then, the two beams that have reflected off the mask are caused to interfere with each other so that an irregularity which constitutes a defect in the mask surface can be detected with a high sensitivity.
In this configuration, the path length of the two sheared optical beam paths becomes long. As a result, a difference between the two branched optical paths can be caused by a disturbance or vibration applied from moving parts. Therefore, the interference intensity of an image plane corresponds to the sum of 1) an optical path difference of the two beams caused by irregularities present on the mask surface, and 2) an optical path difference between the two branched optical paths caused by vibration.
Since the optical path difference between the two branched optical paths caused by vibration fluctuates over time, the interference intensity (brightness of the image) fluctuates over time accordingly. This fluctuation in interference intensity becomes a noise component when detecting defects in the mask surface, so that the defects in the mask surface cannot be detected with a high sensitivity.
Also, the vertical resolution is increased in the shearing direction by applying the two sheared beams of light to an object to be inspected and detecting an interference image. However, the vertical resolution perpendicular to the direction of shearing is not increased. Therefore, the vertical resolution comes to have a directional characteristic, causing such a problem in that the inspection sensitivity varies according to the direction of defects to be inspected.
Further, an object to be inspected may have features produced by a metal wiring process. In metal wiring, fine irregularities called grains are produced in the surface of the object. Since these grains do not affect a device critically, they should not be detected as defects. However, an interference optical system is designed to manifest any irregularity (optical path difference), so that, in the case of a wafer using a metal wiring process, such a manifestation of this type of irregularity by the interference optical system becomes a disadvantage.
It is an object of the present invention to provide an optical system in which optical conditions can be set, such that the contrast of irregularities is kept low in an inspection process where grains exist (example: metal (Al, etc. ) wiring process), and irregularities are manifested in a process when grains do not exist. Further, in order to enhance the inspection sensitivity, not only an increase in vertical resolution by an optical system, but also an increase in horizontal resolution is provided.